Reinforced thin-shell molds

ABSTRACT

A method and a system for forming a polymeric resin mold for molding plastic items, wherein the method includes: (a) using a rapid-prototyping process to create a polymeric resin shell having a configuration in the form of the mold; (b) introducing a reinforcing material into the shell; and (c) curing the shell. The system includes a rapid-prototyping apparatus that creates a plurality of the polymeric resin shells on a platform; a reinforcing material introduction apparatus that introduces a reinforcing material into each of the shells while the shells are on the platform to form a plurality of uncured molds on the platform; and a curing apparatus that cures the shells while they are on the platform.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/775,655,filed Jul. 10, 2007 now U.S. Pat. No. 7,674,442, the disclosure of whichis incorporated herein in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

This disclosure relates to the fabrication of polymeric resin molds usedto form molded plastic items. More specifically, it relates to thefabrication of polymeric resin molds made by a rapid prototypingprocess.

Molded plastic orthodontic alignment appliances (“aligners”) and theirmethod of manufacture as mass-fabricated custom items are disclosed, forexample, in U.S. Pat. No. 5,975,893 and US Patent ApplicationPublication 2005/0082703, both of which are commonly assigned to theassignee of the subject invention, and the complete disclosures of whichare both incorporated herein by reference. Plastic orthodonticappliances, of the type disclosed in the above-referenced documents, aremade as a set of incremental position adjustment appliances that areused to realign or reposition a patient's teeth. The series ofincremental position adjustment appliances is custom made for eachpatient during the fabrication process.

The appliances are typically made by thermal-forming a piece of plasticover a unique mold that represents an incremental position to which apatient's teeth are to be moved, which position corresponds to aparticular incremental position adjustment appliance. The unique moldsare manufactured by downloading digitized representations ofthree-dimensional CAD models of the molds to a rapid prototyping (“RP”)apparatus, such as, for example, stereolithography apparatus (“SLA”) orphotolithography apparatus (“PLA”). Because each aligner is associatedwith a unique mold on which the aligner is fabricated, for the purposesof this disclosure, the molds themselves are considered to bemass-fabricated custom items.

The molding process requires that the molds have substantial structuralrigidity and strength, thereby allowing them to withstand the pressuresand stresses of the molding process without deformation. Thus, the moldshave typically been made as solid forms, each of which is athree-dimensional model or replica of a patient's dental arch at aparticular stage of treatment. The use of solid forms has two distinctdisadvantages: (1) It requires the use of a considerable amount ofpolymeric resin material, which is expensive; and (2) because the RPmachine builds the molds up layer by layer, the greater the volume ofmaterial used in each layer, the longer it takes to form each layer. Tosave on material costs, the completed molds may be hollowed out or“shelled” before they are cured, and the material removed for re-use.There is a limit, however, to the amount of material that can be removedwithout degrading the structural integrity of the molds, and theshelling process itself is costly and time-consuming. Thus, theconventional process for forming the RP molds is costly, both in theexpense of the material and in production time and costs.

It would thus be advantageous to provide a method of making polymericresin molds, such as SLA molds, that can create molds with sufficientstrength and rigidity to be used in the subsequent molding process, andthat can do so while reducing both the amount of polymeric resinmaterial and the fabrication time needed to make the molds.

SUMMARY OF THE INVENTION

As used herein, the terms “the invention” and “the present invention”encompass the invention disclosed herein in its various aspects andembodiments, as well as any equivalents that may reasonably suggestthemselves to those skilled in the pertinent arts.

Broadly, the present invention is a method and a system for forming apolymeric resin mold for molding plastic items, wherein the methodincludes, and the system performs, the steps of: (a) using arapid-prototyping process to create a polymeric resin shell having aconfiguration in the form of the mold; (b) introducing a reinforcingmaterial into the shell; and (c) curing the shell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a platform or tray on which a plurality ofhollow polymeric resin mold shells have been formed by arapid-prototyping process in a rapid-prototyping apparatus;

FIG. 2 is a bottom perspective view of a single mold shell as formed onthe tray of FIG. 1 in accordance with a first embodiment of theinvention;

FIG. 3 is perspective view showing the bottom of the tray of FIG. 1, andshowing also the apparatus for introducing a reinforcing material intothe hollow interior of a mold shell, in accordance with a firstembodiment of the invention;

FIG. 4 is a side elevational view, partly in cross-section, showing theintroduction of a reinforcing material into the hollow interior of amold shell of the type shown in FIG. 2, in accordance with the firstembodiment of the invention;

FIG. 5 is a top perspective view of a single mold shell in accordancewith a second embodiment of the invention;

FIG. 6 is a side elevational view, partly in cross-section, showing theintroduction of a reinforcing material into the hollow interior of amold shell of the type shown in FIG. 5, in accordance with the secondembodiment of the invention; and

FIG. 7 is a diagrammatic representation of a system for formingpolymeric resin molds in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 shows a tray or platform 10,having a top or upper surface 12 on which a plurality of polymeric resinmold shells 14 are formed by a conventional rapid-prototyping process,such as stereolithography (SLA), in a rapid-prototyping apparatus. Themold shells 14 are shown in the form of molds for orthodontic aligners,but they may be configured for molding any desired plastic item. If themold shells 14 are made by the SLA process, they are built up, layer bylayer, on the surface 12 of the platform 10 in an SLA apparatus.

An array of predefined mold locations is defined on the top surface 12of the platform 10, and each of the mold shells 14 is formed at one ofthe predefined mold locations, all of the mold shells 14 being formedsimultaneously at their respective mold locations. An exemplary moldshell 14, in accordance with a first embodiment of the invention, isshown in FIG. 2. The mold shell 14 has a hollow interior 16 and anexterior surface 18 in the configuration of the molded plastic part (inthis case, an orthodontic aligner, not shown) to be formed on the mold.

In the case of mass-customized items, such as orthodontic aligners, eachmold shell 14 is formed in accordance with the information in a uniquetreatment file that identifies the patient, the stage, the associated 3DCAD model, and whether the mold shell is of the upper or lower dentalarch. The treatment file for each mold to be formed on the platform 10is stored in a computer database that may be accessed by the rapidprototyping (“RP”) apparatus either directly or through a local areanetwork (LAN) or any other computer networking system that provides theneeded functionality. In such an application, it is advantageous foreach mold shell 14 to be formed at a predetermined corresponding moldlocation on the platform 10. The specific mold location at which eachmold shell 14 is formed may be either pre-assigned or tracked (afterassignment) by the computer system (not shown) containing the database,and the specific mold location of each mold shell 14 is stored in thedatabase.

After completion of the RP process (e.g., stereolithography orphotolithography), the tray or platform 10, with the mold shells 14formed on its upper surface 12, is now accessed by a reinforcingmaterial introduction apparatus, whereby a reinforcing material isintroduced into the interior of the mold shells 14. The reinforcingmaterial introduction apparatus, an exemplary embodiment of which isshown in FIGS. 3 and 4, may be incorporated in, or be otherwiseassociated with, the RP apparatus, or it may be at a separate location,in which case the platform 10 is moved (e.g., by a conveyor, not shown)to the reinforcing material introduction apparatus. The mold shells 14are too thin and fragile to be removed from the platform 10, so thereinforcing material must be introduced while the shells 14 are still onthe platform 10. In the embodiment illustrated in FIGS. 3 and 4, theplatform 10 has a bottom surface 20 and an array of apertures 22 thatextend through the platform 10 from the bottom surface 20 to the topsurface 12. Each of the above-mentioned mold locations is located so asto coincide with and be accessed by one of the apertures 22.

The exemplary reinforcing material introduction apparatus, as shown inFIG. 3, is advantageously configured to allow the platform 10 to berotatably mounted to permit the inversion of the top surface 12 and thebottom surface 20. To this end, one end of a rotatable shaft 24 may bereleasably attachable to a side edge 26 of the platform 10 by anattachment mechanism, such as a clamp 27. Preferably, a rotatable shaft24 may be releasably fixed, by a clamp 27, to each of two opposite sideedges 26, as shown in FIG. 3. Alternatively, the platform 10 may beclamped or otherwise releasably secured to a rotatable bed (not shown)to which the shaft or shafts 24 may be fixed. The shaft or shafts 24 maybe rotated by an electric motor (not shown) or any other conventionalmeans, controlled to rotate the shaft or shafts 24 so as to flip theplatform 10 to its upside-down position (FIG. 3) when the filling stepis ready to commence. The resin from which the mold shells 14 are formedadheres to the top surface 12 of the platform 10, thereby allowing theplatform 10 to be inverted while the mold shells 14 remain adhered toits top surface 12.

In accordance with a first embodiment of the invention, a reinforcingmaterial 28 (FIG. 4) is introduced into the hollow interior 16 of eachof the mold shells 14 by a nozzle 30 that is connected by a hose or line32 to a source (not shown), such as a tank, container, or otherreceptacle, containing the reinforcing material. The nozzle 30 has adistal tip 34 configured to fit into the tray aperture 22, and thenozzle itself is movable vertically between a lower or filling positionin which the tip 34 is inserted in the aperture 22, and a raised orstand-by position in which the tip 34 is out of the aperture 22. Thematerial 28 is introduced from the bottom surface 20 of the platform 10after the platform has been inverted, so that the reinforcing material28 is introduced directly into the hollow interior 16 of each mold shell14 through the aperture 22 that registers with the mold location 15 atwhich the shell 14 has been formed. As shown in FIG. 3, there isadvantageously provided a plurality of nozzles 30 arranged to address,simultaneously, a row of mold shells 14. Alternatively, a plurality ofnozzles 30 may be provided in an array corresponding to the array ofmold locations on the platform 10, so that there is a separate nozzlefor each of the mold shells 14 on the platform 10.

The reinforcing material 28 may be a conformal filling material, suchas, for example, a liquid plastic resin (e.g., urethane, ABS, PVC, orepoxy), or it may be a plastic foam, such as urethane, polyurethane,styrene, or polystyrene. Foams advantageous, because they can expand tofill the interior volume of the shell 14, and because they are moreeasily cut after curing than is a hardened plastic resin, making thetrimming of the molds easier. On the other hand, epoxies have theadvantage that even a relatively thin coating of epoxy, sprayed into theshell interior by a modified form of the nozzles 30, may, after curing,provide sufficient structural strength and rigidity to the shell towithstand the molding pressures to which it will be subjected, asdescribed above.

Alternatively, the reinforcing material 28 may be a non-resin bulkmaterial, such as plaster, cement, or a silicone-based putty or gel,which is cured by simply by drying. An advantage of a silicone-basedputty or gel is that a volume of such substance substantially less thanthe interior volume of the shell may lend sufficient rigidity andstrength to the shell for the above-described molding process. Stillanother alternative for the reinforcing material 28 is a material with atemperature-dependent viscosity; i.e., a material that is a liquid whenheated to an elevated temperature, and that thickens to a highly-viscoussemi-liquid or paste when cooled to ambient (“room”) temperature.Examples of such materials are waxes (both petroleum-based and“natural,” such as beeswax), and certain plastic resins, such as“HYDROPLASTIC” brand thermoplastic, available from TAK Systems, ofWareham, Mass.

If the reinforcing material 28 is a conformal filler material, as shown,for example, in FIG. 4, it is injected into each shell 14 so as tosubstantially fill the volume of the hollow interior 16 of the shell 14.The injected volume can be a fixed volume that is substantially the samefor each shell, or it can be varied from shell to shell. The fixedvolume may be derived from a predetermined average shell volume valuethat can be stored in the memory of a computer system (not shown) thatcontrols the mold manufacturing process, and particularly the shellfilling step of that process. Alternatively, the nozzles 30 can bemechanically designed and/or controlled to inject the same predeterminedvolume of filler material 28 into all of the mold shells 14. Using theaverage shell volume value for all of the shells is advantageous when aplastic foam is used as the filler material, since the foam expands tofill the available space in the shell interior, and thus preciselymeasuring the volume of foam delivered to match the interior volume ofthe shell is not needed.

Alternatively, the volume of a conformal filler material 28 injectedinto each shell 14 can be adjusted to the approximate interior volume ofthat shell. This method may be advantageous if a liquid resin is used asthe filler material, since the delivered volume of the filler materialmust be approximately the same as the interior volume of the shell, andthe interior shell volume can vary significantly from shell to shell.This method of filling can be accomplished by retrieving a stored volumevalue from the aforementioned database, which, as mentioned above,includes a treatment file for the custom mold to be made from each ofthe shells 14. This treatment file can be updated with the calculatedinterior volume for each shell, and since the database includes aspecific location on the platform 10 for each of the shells 14, thenozzles 30 can be controlled by the computer system to deliver or injectinto each shell the appropriate volume of filler material in accordancewith the volume value retrieved from the treatment file or another fileor location in the database.

As still another alternative, as mentioned above, certain reinforcingmaterials (e.g., certain epoxies and silicone-based fillers), mayrequire a relatively fixed volume, either because they need only beapplied as a relatively thin coating layer on the interior surface ofthe shells 14, or a volume that is substantially less than the averageinterior volume of the shells. The nozzles 30 and the mechanism forcontrolling the volume of reinforcing material delivered may be readilymodified for such applications, as will be apparent to those skilled inthe pertinent arts.

After the reinforcing material has been introduced into the interior ofthe mold shells 14, as described above, the reinforced shells 14 (whichnow may be considered uncured molds) are then accessed by a curingapparatus to cure the mold shells 14. The curing apparatus may beincorporated in, or otherwise associated with, the reinforcementmaterial introduction apparatus, or it may be at a different location,in which case the platform 10 is moved (e.g., by a conveyor, not shown)from the reinforcement material introduction apparatus to the curingapparatus. Typically, the resin used to form the shells 14 is UV-cured,as is well known. If the reinforcing material 28 is of a type thatrequires curing, it may be cured at this point in the process as well.Depending on the particular type of reinforcing material used, itscuring may be accomplished by chemical reaction, cooling, thermosetting,UV setting, or dehydration. (For the purpose of this description, theterm “curing,” as applied to the reinforcing material 28, includescuring, setting, or hardening by any of the aforementioned processes, orany other equivalent processes, as appropriate for the particularmaterial used.) Alternatively, the shells 14 may be UV-cured before thereinforcing material 28 is introduced, in which case the curing stepperformed after the introduction of the reinforcing material 28 willinvolve the curing or setting of the reinforcing material 28 only. Aftercuring, the reinforced mold shells 14 (now cured molds) are separatedfrom the platform 10 for cleaning, trimming, and other processingpreparatory to being used to mold the plastic items (e.g., aligners).

An alternative procedure for introducing a reinforcing material, as usedin a second embodiment of the invention, is illustrated in FIGS. 5 and6. A mold shell 40, as used in the second embodiment of the invention,has a hollow interior (not shown) and an exterior surface 42 configuredas the item to be molded thereon. The mold shell 40 also has a fillerhole 44 communicating with the hollow interior. As in theabove-described first embodiment, a plurality of mold shells 40 isformed on the top surface 12′ of a tray or platform 10′, each of theshells 40 being formed in a specific, predetermined mold location on thetop surface 12′. The platform 10′ in this embodiment, unlike that of theabove-described first embodiment, need not be rotatable in thereinforcement material introduction apparatus, as each of the moldshells 40 is at least partially filled with the reinforcing material 28by a vertically translatable injection nozzle 30′ with a distal tip thatis insertable from the shell's exterior surface 42 through the fillerhole 44 and into the shell's interior while the mold shells 40 aremaintained in their normal orientation (i.e., “right-side up”). In allother material aspects, the second embodiment of the invention isessentially the same as the above-described first embodiment.

FIG. 7 illustrates diagrammatically a system 100 for carrying out theabove-described process. First, as mentioned above, a rapid-prototypingapparatus 102 is employed to form the polymeric resin mold shells 14 or40 on the platform or tray 10 or 10′ by a rapid-prototyping process,such as stereolithography or photolithography. The tray or platformcarrying the mold shells is then accessed by a reinforcing materialintroduction apparatus 104, as described above and as shown in FIGS. 3and 4, and (in an alternate form) in FIG. 6, by which the reinforcingmaterial is introduced into the interior of the hollow shells 14 or 40to form a plurality of uncured molds on the platform. Finally, the trayor platform carrying the reinforced mold shells (uncured molds) isaccessed by a curing apparatus 106, such as, for example, a UV curingapparatus, as is well-known in the art. Alternatively, the curingapparatus may cure the molds by chemical reaction, thermosetting, ordehydration. After the curing process, the cured and hardened molds maysafely be removed from the platform for further processing as needed ordesired.

As mentioned above, the shells may be cured before being filled, andthen subject to the curing process for the purpose of curing thereinforcing material. In that case, the shells are accessed by thecuring apparatus 106 (or another curing apparatus of the same ordifferent type, depending on the materials to be cured) before beingaccessed by the reinforcing material introduction apparatus 104, andthen the reinforced shells (uncured molds) are accessed by a curingapparatus (e.g., the curing apparatus 106) a second time for curing thereinforcing material.

While preferred embodiments of the invention are described above and areillustrated in the drawings, it is understood that these embodiments areexemplary only as the currently preferred embodiments of the invention.It will be appreciated that a number of variations and modificationswill suggest themselves to those skilled in the pertinent arts. Suchvariations, modifications, and equivalents should be considered withinthe spirit and scope of the invention, as defined in the claims thatfollow.

1. A system for making a plurality of polymeric resin molds, comprising:a platform having a top surface defining an array of pre-defined moldlocations, each of the mold locations registering with a correspondingfilling aperture extending from a bottom surface of said platform tosaid top surface; wherein each of the mold locations registers with andis accessed by at least one of the filling apertures; arapid-prototyping apparatus configured and operable to create aplurality of polymeric resin shells on the top surface of the platform,each of the shells having a configuration in the form of a mold; areinforcing material introduction apparatus configured and operable tointroduce a reinforcing material into each of the shells through acorresponding one of the filling apertures while the shells are on theplatform, thereby forming a plurality of uncured molds on the platform;and a curing apparatus configured and operable to cure the uncuredmolds.
 2. The system of claim 1, wherein the reinforcing materialincludes a liquid polymeric resin, and wherein the curing apparatus isconfigured and operable to cure the resin of the reinforcing material.3. The system of claim 1, wherein the reinforcing material includes aplastic foam material.
 4. The system of claim 1, wherein the reinforcingmaterial introduction apparatus is operable to fill each shell with avolume of reinforcing material that is approximately equal to theinternal volume of the shell.
 5. The system of claim 1, wherein theplatform has an aperture corresponding to the filling aperture of eachof the shells formed on the platform by the rapid prototyping apparatus,and wherein the reinforcing material introduction apparatus includes aninjection nozzle mechanism configured and operable to inject thereinforcing material into each of the shells through its correspondingaperture.
 6. The system of claim 5, wherein the injection nozzlemechanism is configured and located so as to enter the apertures fromthe bottom surface.
 7. The system of claim 4, wherein the reinforcingmaterial introduction apparatus includes 34, further comprising amechanism engaging the platform and operable for inverting the platformbefore the injection nozzle mechanism enters the apertures.
 8. Thesystem of claim 1, wherein each mold in the plurality of molds has aunique configuration, and wherein the rapid-prototyping apparatus isconfigured and operable to create a plurality of unique polymeric resinshells, each of the shells having a configuration in the form of aunique mold in the plurality of molds.
 9. A system for making aplurality of polymeric resin molds, comprising: a platform having afirst surface defining an array of platform mold locations, each of theplatform mold locations corresponding to a stored mold location in adatabase; a rapid-prototyping apparatus configured and operable toretrieve the stored mold locations from the database, and to create aplurality of polymeric resin mold shells on the platform, each of themold shells being located at a platform mold location defined by itscorresponding stored mold location; a reinforcing material introductionapparatus configured and operable to introduce a reinforcing materialinto the mold shells through a filling aperture which extends from thefirst surface through the platform to a bottom surface while the moldshells are on the platform, thereby forming a plurality of uncured moldson the platform; and a curing apparatus configured and operable to curethe uncured molds.
 10. The system of claim 9, wherein the reinforcingmaterial includes a liquid polymeric resin, and wherein the curingapparatus is configured and operable to cure the resin of thereinforcing material.
 11. The system of claim 9, wherein the reinforcingmaterial includes a plastic foam material.
 12. The system of claim 9,wherein each mold in the plurality of molds has a unique configuration,and wherein the rapid-prototyping apparatus is configured and operableto create a plurality of unique polymeric resin mold shells, each of themold shells having a configuration in the form of a unique mold in theplurality of molds.
 13. The system of claim 9, wherein rapid-prototypingapparatus is configured and operable to form each of the mold shellswith a filling aperture, and wherein the reinforcing materialintroduction apparatus includes a nozzle configured to introduce thereinforcing material into each of the mold shells through its fillingaperture.
 14. A system for making a plurality of polymeric resin molds,comprising: a platform having a first surface defining an array of moldlocations; a rapid-prototyping apparatus configured and operable tocreate a plurality of polymeric resin mold shells on the platform,wherein each of the mold shells is located at a corresponding one of themold locations, and wherein each of the mold shells has an interiorvolume defined by a corresponding interior volume value stored in adatabase; a reinforcing material introduction apparatus configured andoperable to retrieve the interior volume values from the database and tointroduce into each of the mold shells, through an aperture whichextends from the first surface through the platform to a bottom surfaceof the platform, while the mold shells are on the platform, a volume ofreinforcing material defined by the corresponding interior volume valuefor the interior of the mold shell, thereby forming a plurality ofuncured molds on the platform; and a curing apparatus configured andoperable to cure the uncured molds.
 15. The system of claim 14, whereinthe reinforcing material introduction apparatus is configured andlocated so as to enter the platform apertures from the bottom surface.16. The system of claim 15, further comprising a mechanism engaged withthe platform and operable to invert the platform to permit entry of thereinforcing material introduction apparatus into the apertures from thebottom surface.
 17. The system of claim 14, wherein rapid-prototypingapparatus is configured and operable to form each of the mold shellswith a filling aperture, and wherein the reinforcing materialintroduction apparatus includes a nozzle configured to introduce thereinforcing material into each of the mold shells through its fillingaperture.
 18. The system of claim 14, wherein each of the platform moldlocations corresponds to a stored mold location in the database, andwherein the rapid-prototyping apparatus is configured and operable toretrieve the stored mold locations from the database, whereby each ofthe polymeric resin mold shells formed on the platform is located at aplatform mold location defined by its corresponding stored moldlocation.